Three Phase Rectifier Circuit With Virtual Neutral

ABSTRACT

A rectifier circuit powers three power conversion modules using a three phase AC input without a neutral connection. The rectifier circuit includes a first bridge rectifier that is connected to a first phase of the three phase AC input and that produces a first rectified waveform. A second bridge rectifier is connected to a second phase of the three phase AC input and produces a second rectified waveform. A third bridge rectifier is connected to a third phase of the three phase AC input and produces a third rectified waveform. A first inductor has one end that is connected to the first bridge rectifier. A second inductor has one end that is connected to the second bridge rectifier. A third inductor has one end that is connected to the third bridge rectifier. Opposite ends of the first, second and third inductors are connected to form a virtual neutral. A protection circuit prevents overvoltage when one of the DC outputs is shorted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/163,907 filed on Nov. 3, 2005, which is a divisional of U.S. patentapplication Ser. No. 10/331,774 filed on Dec. 30, 2002, now U.S. Pat.No. 6,977,445, issued Dec. 2, 2005, which is a continuation-in-partapplication of U.S. patent application Ser. No. 09/993,276 filed Nov.16, 2001, now U.S. Pat. No. 6,501,192, issued Dec. 21, 2002. Thedisclosures of the above applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to rectifier circuits, and moreparticularly to rectifier circuits that rectify three phase AC powersources without a neutral connection.

BACKGROUND OF THE INVENTION

Rectifier circuits are commonly used for converting an alternatingcurrent (AC) signal into a direct current (DC) signal. Applications thatrequire either DC power or AC power at a different frequency initiallyrequire the 50-60 Hz three phase AC power to be rectified. The rectifiedDC power can then be used or processed using power conversion modules.

Some applications require DC power at a higher or lower level than therectified DC voltage. In this situation, a power conversion moduleconverts the DC power to the desired higher or lower DC level. When ACpower at a different frequency or voltage is desired, the rectified DCpower is inverted by a power conversion module to AC at the desiredvoltage or frequency.

In some situations, it is desirable to run a DC-AC power conversionmodule without using a regulated DC power supply. Certain types of powerconversion modules (especially 1 MHz and up) become significantly lessefficient as their DC supply voltage is increased.

There are many applications for power conversion modules that aresupplied by 400VAC (common in foreign countries) or 480VAC (common inthe United States) mains. These applications include RF amplifiers andRF generators. The standard practice for high frequency power conversionmodules is to connect two or more lower voltage power conversion modulesin series. However, if one of the series connected modules fails duringoperation, the whole system fails. It is also difficult to share therectified DC input voltage evenly between the series connected powerconversion modules.

When the AC supply is three phase, three AC signals and a ground and/orneutral are typically provided. When the neutral is available, thevoltages that are delivered to the circuit can be phase-to-phase orphase-to-neutral. In many facilities, however, the neutral connection isnot available. When no neutral connection is available, the voltagesthat are delivered to the circuit can be only phase-to-phase. Thephase-to-neutral voltages are typically lower than the phase-to-phasevoltages by a factor that is equal to √{square root over (3)}. The lowervoltages allow higher efficiency in DC-AC high frequency applicationsand a phase-to-neutral connection would be utilized if availability ofthe neutral connection was guaranteed. Also, there are currentrestrictions for the neutral wires, and any application which utilizes aneutral connection provided by their facility is subject to thoserestrictions.

BRIEF SUMMARY OF THE INVENTION

A rectifier circuit according to the present invention powers threepower conversion modules using a three phase AC input without a neutralconnection. The rectifier circuit includes a first bridge rectifier thatis connected to a first phase of the three phase AC input and thatproduces a first rectified waveform. A second bridge rectifier isconnected to a second phase of the three phase AC input and produces asecond rectified waveform. A third bridge rectifier is connected to athird phase of the three phase AC input and produces a third rectifiedwaveform. A first inductor has one end that is connected to the firstbridge rectifier. A second inductor has one end that is connected to thesecond bridge rectifier. A third inductor has one end that is connectedto the third bridge rectifier. Opposite ends of the first, second andthird inductors are connected to form a virtual neutral.

A first capacitor that is connected across the first bridge rectifierfilters the first rectified waveform and creates a first DC railvoltage. A second capacitor that is connected across the second bridgerectifier filters the second rectified waveform and creates a second DCrail voltage. A third capacitor that is connected across the thirdbridge rectifier filters the third rectified waveform and creates athird DC rail voltage.

The first DC rail voltage feeds a first power conversion module. Thesecond DC rail voltage feeds a second power conversion module. The thirdDC rail voltage feeds a third power conversion module. Two of the first,second, and third power conversion modules remain powered at a decreasedvoltage level when the remaining one of the first, second, and thirdpower conversion modules fails.

The first, second, and third bridge rectifiers include first, second,third, and fourth diodes, each with an anode and a cathode. The anode ofthe first diode is connected to the cathode of the second diode. Thecathode of the first diode is connected to the cathode of the thirddiode. The anode of the second diode is connected to the anode of thefourth diode. The anode of the third diode is connected to the cathodeof the fourth diode. The anode of the first diode of the first bridgerectifier is connected to the first phase of the three phase AC input.The anode of the first diode of the second bridge rectifier is connectedto the second phase of the three phase AC input. The anode of the firstdiode of the third bridge rectifier is connected to the third phase ofthe three phase AC input.

The first inductor is connected to the anode of the third diode of thefirst bridge rectifier. The second inductor is connected to the anode ofthe third diode of the second bridge rectifier. The third inductor isconnected to the anode of the third diode of the third bridge rectifier.

A first resistor is connected across the first bridge rectifier. Asecond resistor is connected across the second bridge rectifier. A thirdresistor is connected across the third bridge rectifier.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram illustrating a rectifier circuitthat is connected to three power conversion modules and a three phase ACpower source without a neutral connection according to the presentinvention;

FIG. 2 is a schematic showing three bridge rectifiers that convert thethree phase AC power source without a neutral connection into threerectified waveforms;

FIG. 3 illustrates a waveform that is produced by the three phases ofthe three phase AC power source;

FIG. 4 illustrates a waveform that is produced by the three bridgerectifiers;

FIG. 5 is a schematic showing bridge rectifiers and capacitors thatconvert the three phase AC power source without a neutral connectioninto three DC rail voltages;

FIG. 6 illustrates the waveform that is produced by the three capacitorsof FIG. 5;

FIG. 7 is a schematic showing an inductor between the bridge rectifiersand the capacitors;

FIG. 8 is a schematic showing an inductor between the phases of thethree phase AC power source and the bridge rectifiers;

FIG. 9 is a schematic illustrating an overvoltage protection circuitaccording to the present invention;

FIG. 10 is a schematic illustrating an alternate embodiment withinductors branching out from a virtual neutral;

FIG. 11 is a functional block diagram of a radio frequency (RF)amplifier according to the prior art;

FIG. 12 is a functional block diagram of multiple RF amplifiersaccording to the prior art; and

FIG. 13 is a functional block diagram of multiple RF amplifiers fed by a3-phase virtual neutral circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, the same referencenumbers will be used in the drawings to identify similar elements.

Referring now to FIG. 1, a power conversion circuit 10 includes arectifier circuit 12 that powers three power conversion modules 14-1,14-2 and 14-3. A three phase AC power source 16 without a neutralconnection is connected to the rectifier circuit 12. The three phase ACpower source 16 outputs three AC sinusoidal output voltages 18-1, 18-2and 18-3. The rectifier circuit 12 converts the AC sinusoidal voltagesinto DC rail output voltages 20-1, 20-2 and 20-3 that are output tothree power conversion modules 14-1, 14-2 and 14-3.

Referring now to FIG. 2, the bridge rectifiers 28-1, 28-2 and 28-3 areconnected to the phases of the three phase AC power source 16. Thephases of the three phase AC power source 16 produce AC sinusoidaloutput voltages 18-1, 18-2 and 18-3. The bridge rectifiers 28-1, 28-2and 28-3 rectify the AC sinusoidal output voltages 18-1, 18-2 and 18-3and produce rectified output voltages 30-1, 30-2 and 30-3. The bridgerectifiers 28-1, 28-2 and 28-3 invert the negative portions of the ACsinusoidal output voltages 18-1, 18-2 and 18-3. This is accomplished bythe specific arrangement of the diodes 32 that make up the bridgerectifiers 28-1, 28-2 and 28-3.

The bridge rectifiers 28-1, 28-2 and 28-3 are comprised of four diodes32-1, 32-2, 32-3 and 32-4, each having an anode and a cathode. The anodeof the first diode 32-1 is connected to the cathode of the second diode32-2. The cathode of the first diode 32-1 is connected to the cathode ofthe third diode 32-3. The anode of the second diode 32-2 is connected tothe anode of the fourth diode 32-4. The anode of the third diode 32-3 isconnected to the cathode of the fourth diode 32-4.

Additionally, the anode of the first diode 32-1 of the bridge rectifiers28-1, 28-2 and 28-3 is connected to one output voltage 18-1, 18-2 and18-3, respectively, of the three phase AC power source 16. A connection34 is made between the bridge rectifiers 28-1, 28-2 and 28-3 that formsa virtual neutral. The virtual neutral connection 34 includes aconductor that connects the anode of the third diode 32-3 of the threebridge rectifiers 28-1, 28-2 and 28-3.

FIG. 3 illustrates an exemplary waveform 42 that is produced by thephases of the three phase AC power source 16. The waveforms of eachphase are offset by 120° from each other. FIG. 4 illustrates anexemplary waveform 50 that is produced by the bridge rectifiers 28-1,28-2 and 28-3. As previously discussed above, the diodes 32 rectify thewaveform 42 of the AC sinusoidal voltages 18-1, 18-2 and 18-3 that areshown in FIG. 3.

Referring now to FIG. 5, capacitors 58 are preferably used to smooth therectified voltage. For purposes of clarity, reference numbers from FIG.2 are used in FIG. 5 to identify similar elements. The bridge rectifiers28-1, 28-2 and 28-3 and capacitors 58 convert the AC sinusoidal outputvoltages 18-1, 18-2 and 18-3 from the three phase AC power source 16into DC rail output voltages 20-1, 20-2 and 20-3. The capacitors 58filter the rectified voltages 30-1, 30-2 and 30-3 by smoothing thevoltage peaks that are shown on the waveform 50 of FIG. 4. The result isan approximately constant output voltage. The DC rail output voltages20-1, 20-2 and 20-3 of the rectifier circuit 12 are of substantiallyequal magnitude. Additionally, the DC rail output voltages 20-1, 20-2and 20-3 feed power conversion modules 14-1, 14-2 and 14-3. If any oneof the three power conversion modules 14-1, 14-2 and 14-3 fail duringoperation, the remaining two power conversion modules 14-1, 14-2 and14-3 will remain powered at a decreased voltage level.

FIG. 6 illustrates the waveform 66 that is produced by the capacitors58. A plot of the DC rail voltages 20-1, 20-2 and 20-3 as a function oftime is shown. As discussed above, the capacitors 58 filter therectified output voltages 30-1, 30-2 and 30-3 from the bridge rectifiers28-1, 28-2 and 28-3 by smoothing the peak voltages of the waveform 50 ofFIG. 4.

Referring now to FIG. 7, inductors 74 can be optionally located betweenthe bridge rectifiers 28-1, 28-2 and 28-3 and the capacitors 58 tofurther smooth the DC rail voltages 20-1, 20-2 and 20-3 that areproduced by the capacitors 58. The inductors 74 decrease the ripples inthe DC rail voltages 20-1, 20-2 and 20-3 that are produced by thecapacitors 58 and that are shown in FIG. 6. Additionally, the inductors74 increase the power factor.

In FIG. 8, inductors 82 between the phases of the three phase AC powersource 16 and the bridge rectifiers 28-1, 28-2 and 28-3 smooth the DCrail voltages 20-1, 20-2 and 20-3 that are filtered by the capacitors58. The inductors 82 perform a similar function to the inductors 74 ofthe schematic in FIG. 7. The inductors 82 decrease the ripples in the DCrail output voltages 20-1, 20-2 and 20-3 that are produced by thecapacitors 58 and increase the power factor of the rectifier circuit.

Referring now to FIG. 9, when one of the three DC outputs is shorted,the other two DC output voltages will increase. An overvoltageprotection circuit 100 according to the present invention is used toprotect the loads from overvoltage. The overvoltage protection circuit100 includes a capacitor C1 that is connected in parallel with aresistor R1. One end of the resistor R1 is connected to a cathode of azener diode CR1. An opposite end of the zener diode CR1 is connected toone end of a resistor R2 and a gate of a silicon controlled rectifier(SCR) CR2. An opposite end of the resistor R2 is connected to anopposite end of the resistor R1 and a cathode of the zener diode CR2. Ananode of the zener diode CR2 is connected to one end of a transzorb CR3.An opposite end of the transzorb CR3 is connected to the cathode of CR2and to an emitter of a transistor Q1. In a preferred embodiment, thetransistor Q1 is an insulated gate bipolar transistor (IGBT).

A gate of the transistor Q1 is connected to one end of a third resistorR3, to the one end of the transzorb CR3, and to the anode of the zenerdiode CR2. An opposite end of the resistor R3 is connected to thecathode of the zener diode CR1, the one end of the resistor R1, to oneend of the capacitor C1, and to a positive terminal of the load. Acollector of the transistor Q1 is connected to a negative terminal ofthe load.

In use, the capacitor C1 and the resistor R1 are the capacitor bank andbleeder resistor, which are located at the output of each of the threevirtual neutral DC outputs. When the voltage across the capacitor C1exceeds the voltage rating of the zener diode CR1, the zener diode CR1begins to conduct current. A voltage drop is developed across theresistor R2. The gate of the SCR CR2 is biased and the SCR CR2 istriggered to an on state. The gate of the transistor Q1 is clamped toits emitter by the SCR CR2 and the transistor Q1 turns off, whichprotects the load. The resistor R3 provides a gate bias for thetransistor Q1 during normal operation and the transzorb CR3 limits thegate-source voltage to a safe level.

Referring now to FIG. 10, an alternate method for forming a virtualneutral and connecting inductors is shown. The circuit 120 includes avirtual neutral 124 and inductors 126-1, 126-2 and 126-3 (collectivelyreferred to as 126). Capacitors 58-1, 58-2 and 58-3 and resistors 128-1,128-2, and 128-3 are connected across the load. The inductors 126 branchout from the virtual neutral and provide power factor correction. Inaddition, the inductors 126 reduce shifting of the virtual neutral pointwhen the three loads are not equal. In other words, the DC outputvoltages are closer to each other even when different current levels aresupplied to the three loads.

Referring now to FIG. 11, a radio frequency (RF) amplifier circuit 150according to the prior art is shown. The RF amplifier circuit 150includes a three-phase rectifier circuit 152 that receives a 3-phase ACinput voltage. The rectifier circuit 152 outputs an unregulated DCoutput voltage to a DC-DC converter 154. The DC-DC converter convertsthe unregulated DC output voltage to a regulated DC output voltage thatis input to a DC-RF converter or RF power amplifier 156. While examplevalues are shown in FIG. 11, skilled artisans will appreciate that otherDC and AC voltage values may be used. Referring now to FIG. 12, amultiple amplifier circuit 160 includes three RF power amplifiers 160-1,160-2 and 160-3 that output RF output signals to a 3-way combiner 170that generates a combined RF output.

Referring now to FIG. 13, a multiple RF amplifier circuit 200 with the3-phase virtual neutral circuit according to the present invention isshown. The multiple RF amplifier circuit 200 includes a 3-phase virtualneutral circuit 202, which receives a three phase AC input voltage andoutputs three floating, regulated DC output voltages to RF poweramplifiers 204-1, 204-2 and 204-3. RF outputs of the RF power amplifiers204-1, 204-2 and 204-3 are input to a three-way combiner, whichgenerates a combined RF output. The multiple RF amplifier circuit 200according to the present invention eliminates the need for the DC-DCconverter 154 in FIG. 12, which reduces cost and improves reliability.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

1. A radio frequency (RF) power amplifier circuit comprising: a virtualneutral circuit that forms a virtual neutral and that receives athree-phase AC input voltage; a first RF power amplifier thatcommunicates with said virtual neutral circuit and that generates afirst RF output; a second RF power amplifier that communicates with saidvirtual neutral circuit and that generates a second RF output; a thirdRF power amplifier that communicates with said virtual neutral circuitand that generates a third RF output; and a combiner that combines saidfirst, second and third RF outputs into a combined RF output.
 2. The RFpower amplifier circuit of claim 1 wherein said virtual neutral circuitincludes: a first bridge rectifier that is connected to a first phase ofsaid three phase AC input and that produces a first rectified waveform;a second bridge rectifier that is connected to a second phase of saidthree phase AC input and that produces a second rectified waveform; anda third bridge rectifier that is connected to a third phase of saidthree phase AC input and that produces a third rectified waveform. 3.The rectifier circuit of claim 2 further comprising: a first capacitorthat is connected across said first bridge rectifier that filters saidfirst rectified waveform and that creates a first DC rail voltage; asecond capacitor that is connected across said second bridge rectifierthat filters said second rectified waveform and that creates a second DCrail voltage; and a third capacitor that is connected across said thirdbridge rectifier that filters said third rectified waveform and thatcreates a third DC rail voltage.
 4. The rectifier circuit of claim 3wherein said first, second, and third DC rail voltages are of equalmagnitude.
 5. The rectifier circuit of claim 3 wherein said first DCrail voltage feeds a first power conversion module, said second DC railvoltage feeds a second power conversion module, and said third DC railvoltage feeds a third power conversion module.
 6. The rectifier circuitof claim 5 wherein two of said first, second, and third power conversionmodules remain powered at a decreased voltage level when the remainingone of said first, second, and third power conversion modules fails. 7.The rectifier circuit of claim 2 further comprising: a first inductorthat is connected between said first bridge rectifier and said firstcapacitor that smoothes said first DC rail voltage and increases a firstpower factor; a second inductor that is connected between said secondbridge rectifier and said second capacitor that smoothes said second DCrail voltage and increases a second power factor; and a third inductorthat is connected between said third bridge rectifier and said thirdcapacitor that smoothes said third DC rail voltage and increases a thirdpower factor.
 8. The rectifier circuit of claim 2 further comprising: afirst inductor that is connected between said first phase of said threephase AC input and said first bridge rectifier that smoothes said firstDC rail voltage and increases a first power factor; a second inductorthat is connected between said second phase of said three phase AC inputand said second bridge rectifier that smoothes said second DC railvoltage and increases a second power factor; and a third inductor thatis connected between said third phase of said three phase AC input andsaid third bridge rectifier that smoothes said third DC rail voltage andincreases a third power factor.
 9. The rectifier circuit of claim 2wherein said first, second, and third bridge rectifiers include: first,second, third, and fourth diodes, each with an anode and a cathode,wherein said anode of said first diode is connected to said cathode ofsaid second diode, said cathode of said first diode is connected to saidcathode of said third diode, said anode of said second diode isconnected to said anode of said fourth diode, said anode of said thirddiode is connected to said cathode of said fourth diode, said anode ofsaid first diode of said first bridge rectifier is connected to saidfirst phase of said three phase AC input, said anode of said first diodeof said second bridge rectifier is connected to said second phase ofsaid three phase AC input, and said anode of said first diode of saidthird bridge rectifier is connected to said third phase of said threephase AC input.
 10. The rectifier circuit of claim 9 wherein saidvirtual neutral includes a conductor connecting said anode of said thirddiode of said first bridge rectifier, said anode of said third diode ofsaid second bridge rectifier, and said anode of said third diode of saidthird bridge rectifier.
 11. The rectifier circuit of claim 2 furthercomprising: a first inductor having one end that is connected to saidfirst bridge rectifier; a second inductor having one end that isconnected to said second bridge rectifier; and a third inductor havingone end that is connected to said third bridge rectifier, whereinopposite ends of said first, second and third inductors are connected toform said virtual neutral.